Obesity is one of the greatest risk factors for disease and death in our modern society. People with obesity are at a higher risk of heart disease, type 2 diabetes, some cancers and premature death. Therefore, the question has been raised – does deliberate weight loss reduce the risk of mortality from these conditions?
Studies that explore associations are unable to demonstrate whether deliberate weight loss can reduce the risk of overall mortality, cardiovascular disease or cancer.
Although many research studies have explored the connection between obesity, weight loss and mortality in different areas, it needs to be collated to examine the relationships. By bringing high-quality research together to analyse, researchers can draw conclusions about the impact of weight loss on mortality risk.
Researchers conducted a systematic review and meta-analysis of randomised controlled trials, or RCTs.
RCTs that were included were conducted on adults, with a minimum follow-up term of 1 year. Participants had an average BMI of ≥30 at the start of the RCT. The RCT had to have a focus on weight loss using a weight-reducing diet compared to a control group.
The RCTs could also have advice for increasing physical activity or a physical activity program incorporated. Trials with pregnant or postpartum women were among those excluded.
RCTs were sourced from numerous databases, including Medline, Cochrane Central Register of Controlled Trials and Embase.
54 RCTs were identified in the search, with a total of 30,206 participants. All the trials except one evaluated diets that were low-fat and weight-reducing.
The evidence supported weight-loss interventions for decreasing all-cause mortality, with an 18% reduction of premature mortality risk over the median trial length of 2 years. This equated to six fewer deaths per 1000 participants.
Moderate-quality evidence supported an effect on mortality from cardiovascular events. There was very low-quality evidence to support an effect on cancer mortality.
Weak evidence supported long-term weight loss in trials that provided structured physical activity programs compared to those with physical activity advice or no advice.
Weight reducing diets may reduce premature mortality from all causes in adults with obesity. The diets most supported by the evidence are diets that are low in fat and saturated fat. Physical activity programs may support long-term weight loss efforts.
Evidence is stronger for all-cause mortality and cardiovascular events, and weaker evidence supports an impact on cancer mortality.
However, researchers noted that they did not have evidence to prove or dispute whether other diet options may have an equal or greater impact on any of the mortality outcomes. There was also insufficient data to explore whether sub-groups responded differently to the interventions. More research is required to explore these questions further.
Ma Chenhan, Avenell Alison, Bolland Mark, Hudson Jemma, Stewart Fiona, Robertson Clare et al. Effects of weight loss interventions for adults who are obese on mortality, cardiovascular disease, and cancer: systematic review and meta-analysis, BMJ 2017; 359 :j4849